This invention relates to a process for separating solid particles from liquid nitrogen, as well as to an apparatus for carrying out the process and also relates to a refrigerator including the apparatus.
Liquid nitrogen is used to freeze or cool some kind of medicines or foods. Normally, crude liquid nitrogen contains undesirable impurities, i.e., various potentially deadly germs and other undesirable particles of various sizes. Therefore, liquid nitrogen for such special use should be purified.
Endotoxin is regarded to the smallest particle among the undesirable impurities. Endotoxin comprises mainly lipoic-polysaccharide which is a main component of the cell membrane of gram-negative bacteria. The "gram-negative" bacteria indicates bacteria which shows a red color after Gram's stain classification method. If a medical procedure, such as a blood transfusion, injection, or an intravenous drip is contaminated with endotoxin, the endotoxin may cause fever or shock as a harmful side effect. The endotoxin is also known to be a representative pyrogen (a material which causes fever when it is introduced into a patient's body). It is important for the following points:
i) Air or water contains gram-negative bacteria at relatively higher concentrations than other bacteria. Thus, bacteria involving endotoxin is introduced to the patient with relatively high probability.
ii) Endotoxin causes fever even with a very small quantity. The concentration limit for a patient is 1 nanogram per kilogram (ng/kg) for intraveneous injection.
iii) Even if the body of bacteria is destroyed or the medical apparatus is sterilized, the activity of endotoxin is not reduced.
iv) Endotoxin is relatively stable against heat.
Thus, it is desirable to remove undesirable impurities, such as endotoxin, from liquid nitrogen (-196.degree. C.). Generally, the removing process includes vaporizing liquid nitrogen, filtering the nitrogen gas with an organic film filter device and re-liquefying the nitrogen gas into liquid. The filtrated liquid nitrogen can be used to freeze or cool medicines or foods.
Since in the conventional methods of removing impurities, the liquid nitrogen is first vaporized into a gas and then re-liquified, filtration of liquid nitrogen is expensive and also inefficient from industrial point of view.
In addition, the organic film filter is inferior in chemical resistance, mechanical strength and lasting quality.
It is thus desirable to provide, a process and an apparatus which can filtrate nitrogen while in the liquid state without vaporization.